Crystal structure of 4′-{[4-(2,2′:6′,2′′-terpyridyl-4′-yl)phenyl]ethynyl}biphenyl-4-yl (2,2,5,5-tetramethyl-1-oxyl-3-pyrrolin-3-yl)formate benzene 2.5-solvate

The title compound, C₄₄H₃₅N₄O₃·2.5C₆H₆ (1), consists of a terpyridine and a N-oxylpyrroline-3-formate group separated by an aromatic spacer, viz. 4-(phenylethynyl)-1,1′-biphenyl. It crystallized in the triclinic space group P-1 with two and a half benzene solvate molecules (one benzene molecule is located about an inversion center), while the dichloromethane solvate (2) of the same molecule [Ackermann et al. (2015). Chem. Commun. 51, 5257-5260] crystallized in the tetragonal space group P42/n, with considerable disorder in the molecule. In (1), the terpyridine (terpy) group assumes an all-trans conformation typical for terpyridines. It is essentially planar with the two outer pyridine rings (B and C) inclined to the central pyridine ring (A) by 8.70(15) and 14.55(14)°, respectively. The planes of the aromatic spacer (D, E and F) are nearly coplanar with dihedral angles D/E, D/F and E/F being 3.42(15), 5.80(15) and 4.00(16)°, respectively. It is twisted with respect to the terpy group with, for example, dihedral angle A/D being 24.48(14)°. The mean plane of the N-oxylpyrroline is almost normal to the biphenyl ring F, making a dihedral angle of 86.57(16)°, and it is inclined to pyridine ring A by 72.61(15)°. The intramolecular separation between the O atom of the nitroxyl group and the N atom of the central pyridine ring of the terpyridine group is 25.044(3)Å. In the crystal, molecules are linked by pairs of C - H⋯O hydrogen bonds, forming inversion dimers. The dimers stack along the c axis forming columns. Within and between the columns, the spaces are occupied by benzene molecules. The shortest oxygen-oxygen separation between nitroxyl groups is 4.004(4)Å. The details of the title compound are compared with those of the dichloromethane solvate (2) and with the structure of a related molecule, 4′-{4-[(2,2,5,5-tetramethyl-N-oxyl-3-pyrrolin-3-yl)ethynyl]phenyl}-2,2′:6′,2′′-terpyridine (3), which has an ethynylphenyl spacer [Meyer et al. (2015). Acta Cryst. E71, 870-874].